Petrologic significance of Fe-rich staurolite in pelitic schists of the Silgará Formation, Santander Massif

Abstract

Medium grade metapelites of the Silgará Formation at the Santander Massif (Colombian Andes) have been affected by a medium-pressure/high-temperature Barrovian type of metamorphism, developing a sequence of metamorphic zones (biotite, garnet, staurolite and sillimanite). These rocks record a complex tectonometamorphic evolution and reaction history. Metapelitic rocks from the staurolite zone are typically foliated, medium-to coarse-grained, pelitic to semipelitic schists that contain the mineral assemblage biotite + garnet + staurolite ± kyanite; all contain muscovite + quartz + plagioclase with minor K-feldspar, tourmaline, apatite, zircon, epidote, calcite, and Fe-Ti oxides. Field and microscopic evidences reveal that Fe-rich staurolite in pelitic schists is involved in several chemical reactions, which explains its formation and transformation to other minerals, which are very important to elucidate the reaction history of the Silgará Formation metapelites.Medium grade metapelites of the Silgará Formation en el Macizo de Santander (Andes Colombianos) han sido afectadas por un metamorfismo de tipo Barroviense, el cual se ha producido en condiciones de media presión y alta temperatura, desarrollando una secuencia de zonas metamórficas (biotita, granate, estaurolita y silimanita). Estas rocas registran una evolución tectono-metamórfico e historia reacción compleja. Las metapelitas de la zona de la estaurolita están representadas por esquistos pelíticos a semipelíticos de grano medio a grueso típicamente foliados que contienen la paragénesis mineral biotita + granate + estaurolita ± cianita; todos contienen moscovita + cuarzo + plagioclasa con menor feldespato potásico, turmalina, apatito, zircón, epidota, calcita, y óxidos de Fe-Ti. Evidencias de campo y microscópicas revelan que la estaurolita rica en Fe en esquistos pelíticos se involucró en varias reacciones químicas a partir de las cuales esta se formó o transformó a otra fase mineral, lo cual es muy importante para dilucidar la historia de reacción de los metapelitas de la Formación Silgará. ABSTRACT RESUMEN

Characterization of a chiastolite-type andalusite: structure and physicochemical properties related to mullite transformation

“…Simultaneously, most of the green round-like Si element is distributed in the Al zone with some coarse meshes of approximately 50 μm, which are not easily observed by SEM. In micaceous metamorphic rocks, Ríos et al [25] highlighted that Al-rich minerals such as andalusite form poikiloblasts rich in quartz inclusions. EDS analysis also shows the existence of Fe (in yellow colour) in relatively small quantities, while the distribution of K and Mg elements distribution exhibits a complementary relationship, suggesting that Fe, K, and Mg may attest to the existence of the muscovite's phase [19].…”